Conventional concrete, consisting of hydrated Portland cement with silica sand as fine aggregate and limestone, stone or other coarse aggregate, is a good electrical insulator. The electrical resistivity of concrete usually ranges from 6.54.times.10.sup.5 .OMEGA.cm to 11.4.times.10.sup.5 .OMEGA.cm for dried concrete and 2.5 to 4.5.times.10.sup.3 .OMEGA.cm for moist concrete. There are several uses, however, for a concrete that is electrically conductive. One of these uses is electromagnetic shielding. It is often required in the design and construction of facilities and equipment to protect electrical systems or electronic components from the effects of unwanted electromagnetic energy. Other applications are radiation shielding in the nuclear industry; antistatic flooring in the electronic instrumentation industry and hospitals; and cathodic protection of steel reinforcement in concrete structures.
These needs have been recognized for a number of years and some conductive concrete compositions and articles have been described in the technical and patent literature. Banthia et al, Cem. Concr. Res., 22(5), 804-814 (1992), studied the electrical resistivity of carbon fiber and steel micro-fiber reinforced cements. The content of conductive fiber ranged from 1 to 5% by volume. The resistivity at 28 days of hydration ranged from 78 .OMEGA.cm to 31.92 k.OMEGA.cm.
Kojima et al, CAJ Proceedings of Cement & Concrete, The Cement Association of Japan, No. 43, 560-565 (1989), prepared a highly conductive carbon fiber/cement composite by laminating six sheets of carbon fiber paper impregnated with Portland cement paste. The product was 3 mm thick and had a resistivity value of 0.7 .OMEGA.cm. The material was highly effective in electromagnetic shielding, but very expensive and not suitable for load-bearing applications.
Chiou et al, Composites, 20(4), 379-381 (1989), reported work on carbon fiber reinforced cement for electromagnetic shielding, with results resembling those of Banthia et al.
Japanese patent application No. (Kokai) JP 63,215,542, Akira et al, describes fiber-reinforced conductive concrete articles, useful for anti-static flooring, as comprising conductive fibers, reinforcement fibers, cement aggregates and synthetic resins or rubbers. Thus, a mixture of 60 parts EVA, 120 parts water, water-reducing agent and surfactant was mixed with 14 parts PAL fiber (alkali-resistant glass fiber) and kept still for at least 2 weeks. The supernatant of the solution was mixed with carbon fiber (3 parts), portland cement (400 parts) and silica sand (300 parts), and then the precipitated glass fiber was added to give a mixture which was cured 7 days to give a test piece having surface resistivity (JIS K611) 1.8.times.10.sup.6 .OMEGA.cm and abrasion loss (JIS A1451) of 0.30 mm.
Katsumi et al, Japanese Application (Kokai) JP 89 14,137 describe electrically conductive concrete containing chopped strands of curved C (carbon) fibers having aspect ratio.gtoreq.50 and a higher specific volume than straight carbon fibers. A mixture of Portland cement 100, Donacarbo S-231 (curved C fibers, aspect ratio 222, sp. vol. 11 cm.sup.3 /g) 2, sand 150, water 70, polyacrylic acid ester 15, and naphthalenesulfonic acid-formalin condensate (15 parts) was molded and hardened to give a concrete sample having intrinsic volume resistivity 7.5 .times.10.sup.3 and 5.2.times.10.sup.6 .OMEGA.cm in the longitudinal and lateral direction, respectively, vs. 6.9.times.10.sup.6 and 3.1.times.10.sup.7 .OMEGA.cm for one with straight C fibers.
Hideo, Japanese Application (Kokai) No. 89 72,947, proposed an electrically conductive cement composition containing C fibers and an alkali metal carbonate and/or hydrocarbonate. The C fibers are uniformly dispersed in the cement powder. The cement compositions have high electrical conductivity and are useful as grounding resistance-decreasing and radiation absorbing materials. According to the specification, a mixture containing portland cement 100, C fibers 2, and NaHCO.sub.3 2 wt. parts, was mixed with 80 wt. parts water, and the resulting mixture was used as a grounding resistance-decreasing agent for encapsulating a Cu lightning rod in the ground.
Toshio et al, Japanese Kokai No. 89 126,248, describe electroconductive cement compositions containing acetylene black. Mortar prepared from Portland cement 100, sand 300, partially oxidized acetylene black 50 and water 90 parts, gave concrete having electrical resistance from 7 days to 6 mo after curing 30 .OMEGA., and after 1 year 25 .OMEGA., and flexural and compressive strengths 43 and 190 kgcm.sup.2, respectively.
Hideo et al described a number of electroconductive cement compositions, Japanese Kokai No. No. 89 131,042; 89 141,852; and 89 141,853.
Other prior art compositions have been described by: Junichi et al, Kokai 89 234,347; Tadashi et al, Kokai 88 215,544; Tadayuki et al, Kokai 90 172,846; Chinese Patent 1,048,696 to Xia Guirong et al; Japanese Kokai No. 92 02,637 (Hiroharu et al); German OS 4,201,871 (W. Jaschinski et al); and Japanese Kokai No. 92 74,747 (Y. Sasagawa et al).
While these prior art compositions may be useful for specific purposes, it is noted that, where sufficient details are available, a compromise between high mechanical strength and relatively low electrical resistivity has not been achieved by any of the subject compositions. Many of the prior art conductive concrete (or mortar) recipes use a significant amount of carbon fiber which is relatively expensive.
Accordingly, there is still a need for low-cost conductive concrete compositions combining good mechanical strength and high electrical conductivity.